First and foremost, be careful. The country is in quite a rough situation right now, but I'm sure that you're aware of this. In the highly unlikely case that you're not, I urge you to check BBC for some info on the recent political and military happenings.
Also, I would love to see pictures, and read stories, give us a feel for how it was when you come back!
Take in the sights, it's quite a gorgeous country.
I will ask my Sri-Lankan friend for some useful Sinhalese phrases, and try to write down the correct pronunciation for you.
Actually...here:
Sinhala Phrases